1. Field of the Invention
The present invention relates generally to laser treatment of soft tissue, and more particularly to laser treatment of the prostate.
2. Description of the Prior Art
Benign Prostatic Hyperplasia (BPH) is a condition wherein continued growth of the prostate restricts the passage of urine through the lower portion of the bladder and the urethra. BPH is often treated by surgically removing excess prostate tissue from the transitional zone of the prostate that is pressing on the urethra, which usually relieves the bladder outlet obstruction and incomplete emptying of the bladder caused by the BPH.
Recently, the most commonly employed procedure for removal of excess prostate tissue has been transurethral resection of the prostate, also known as TURP. In the TURP procedure, the surgeon utilizes a standard electrical cutting loop to shave off small pieces of the targeted tissue from the interior of the prostate. At the end of the operation, pieces of excised prostate tissue are flushed out of the bladder using an irrigant.
While effective, the TURP procedure is known to cause numerous side effects, including incontinence, impotence, retrograde ejaculation, prolonged bleeding and TUR syndrome. Recently, alternative procedures have been developed which reduce or avoid the side effects associated with TURP. One class of procedures involves xe2x80x9ccookingxe2x80x9d prostate tissue by heating it to a to a temperature above 45 degrees Celsius. Typically this is accomplished using electrically resistive elements such as: radio frequency (RF), microwave, or long-wavelength lasers. An example of a procedure of this nature is discussed in U.S. Pat. No. 6,064,914 by Trachtenberg (xe2x80x9cThermotherapy Methodxe2x80x9d). Because these procedures leave the thermally-treated tissue in place, post-procedure edema, dysuria, and retention rates are relatively high. Further, use of thermal procedures requires the patient to be catheterized for several days following the procedure, and may cause extensive and unpredictable scarring of the intra prostatic urethra.
Another class of procedures involves vaporizing or ablating the targeted tissue using laser light. These procedures generally avoid the high infection rates and scarring problems of thermally-based procedures. However, laser ablation of prostate tissue has to date, required the use of an expensive laser capable of generating high-power laser light. The high cost of purchasing or leasing such a laser results in a concomitant increase in the cost of the procedure. Finally, the ablation process typically occurs slowly, resulting in a lengthy procedure time.
The Ho:YAG laser and its fiberoptic delivery system is an example of a laser that is commonly used for ablating prostate tissue. The Ho:YAG laser generates pulses of 2100 nm light that are strongly absorbed by water in the prostate tissue and in the saline irrigant positioned between the distal end if the fiberoptic and the tissue. The absorption coefficient of water is so high at 2100 nm that 50% of the light is absorbed within 0.2 mm. Consequently even a thin layer of irrigant positioned between the distal end on the fiberoptic and the tissue will absorb a large fraction of the laser light. Furthermore with the short pulse durations (Tp less than 0.5 ms) and large pulse energies (Ep greater than 1.0 joule) used for ablating prostate tissue the irrigant is explosively boiled creating a shock wave that tears tissue. Because water is such a large constituent of prostate tissue and blood, there is essentially no selective absorption by blood. This combination of violent tissue disruption and the superficial unselective light penetration leads to poor hemostasis.
Nd:YAG lasers operating at 1064 nm have also been used for ablating prostate tissue. Although 1064 nm light is hemostatic at high power levels its low absorption in blood and prostate tissue leads to inefficient ablation and a large residual layer of thermally denatured tissue several millimeters thick. After surgery the thermally denatured tissue swells and leads to transient urinary retention, which can cause long catheterization times, painful urination, and high infection rates.
Frequency doubled Nd:YAG lasers operating at 532 nm in a Quasi continuous mode at power levels up to 60 watts have been used to efficiently and hemostatically ablate prostate tissue. These lasers are pumped by CW krypton arclamps and produce a constant train of Q-switched pulses at 25 kHz. The high Q-Switch frequency makes the tissue effects indistinguishable from CW lasers of the same average power. The 532 nm light from these lasers is selectively absorbed by blood leading to good hemostasis. When ablative power densities are used, a superficial layer of denatured prostate tissue less than 1 mm is left behind. This thin layer of denatured tissue is thin enough that the immediate post surgical swelling associated with other treatment modalities is greatly reduced. This reduced swelling leads to short catheterization times and less dysuria. At high powers, 532 nm lasers induce a superficial char layer (an absorptive, denatured layer) that strongly absorbs the laser light and greatly improves the ablation efficiency. The problem with the existing 532 nm lasers used to date is that they are large, expensive, inefficient, and have a highly multi-mode output beam that makes them inefficient for ablating prostate tissue.
High power densities are required for rapid and efficient vaporization of prostate tissue. The difficulty of achieving higher average output power densities is that when high input powers are supplied to the laser element from an excitation source such as an arclamp a large amount of heat is generated in the lasing element. This heat induces various deleterious effects in the lasing element. In particular the temperature difference between the coolant and the hot lasing element generates a thermally induced graded index lens that decreases the beam quality of the laser and causes the laser to operate with more transverse optical modes than it would otherwise.
The M2 parameter is a well established convention for defining the beam quality of a laser and is discussed in pages 480-482 of Orazio Svelto and David C. Hanna, Principles of Lasers, Plenum Press, New York, 1998, which is incorporated herein by reference. The beam quality measures the degree to which the intensity distribution is Guassian. The quantity M2 is sometimes called inverse beam quality rather than beam quality but in this application it will be referred to as beam quality. M2 is defined as                     M        x        2            ≡                                    (                                          σ                x                            ⁢                              σ                f                                      )                    NG                                      (                                          σ                x                            ⁢                              σ                f                                      )                    G                      =          4      ⁢              xe2x80x83            ⁢      π      ⁢              xe2x80x83            ⁢                        (                                    σ              x                        ⁢                          σ              f                                )                NG              ,
where xcfx80 refers to the number 3.14 . . . , "sgr" is used to represent the spot size, the subscripts x and f represent the spatial and frequency domains along the x-axis, respectively, and the subscripts G and NG signify Guassian and non-Guassian, respectively. The x-axis is transverse to the direction of propagation of the beam. The beam quality in any direction transverse to the beam may be essentially the same. Therefore the subscript x is dropped from the M2 elsewhere in the specification. The beam widths or "sgr"s are determined based on the standard deviation of the position, where the squared deviation of each position is weighted by the intensity at that point. The beam width in the frequency domain "sgr"f is the beam width of the beam after being Fourier transformed.
The formula usually used for calculating the angular divergence, xcex8, of a beam of light of wavelength xcex is strictly valid only for a beam having a Guassian intensity distribution. The concept of beam quality facilitates the derivation of the angular divergence, xcex8, for the beam with a non-Guassian intensity distribution, according to   θ  =                    M        2            ⁢              (                              2            ⁢                          xe2x80x83                        ⁢            λ                                π            ⁢                          xe2x80x83                        ⁢                          σ              x                                      )              .  
For example, a TEM00 laser beam has a high beam quality with an M2 of 1, whereas by comparison, high power surgical lasers operate with M2 values greater than 100.
The Applicants have recognized that high power lasers typically have an M2 greater than 144. The larger number of modes makes M2 larger and makes it difficult to focus the light into small, low numerical aperture fibers and reduces the ability to project high power density light onto tissue. As a result, the vaporization efficiency of CW arclamp pumped 532 nm lasers on prostate tissue is significantly reduced.
According to one embodiment of the invention, a method for treating BPH comprises the steps of providing a solid-state laser having a laser element positioned to receive pump radiation from an excitation source; in some cases modulating the source to cause the laser to emit pulsed laser light; and delivering the laser light to targeted tissue. Various solid-state lasers may be used for this purpose, including (without limitation), a Q-switched arc lamp-pumped or a flash lamp-pumped laser using a frequency doubling crystal such as potassium-titanyl-phosphate (KTP). The pulse duration of the laser light is preferably in the range of 0.1 to 500 milliseconds, and the wavelength of the laser light is preferably between 200 and 1000 nanometers. The laser light is preferably delivered to the targeted prostate tissue through an optical fiber terminating at a distal end in a side-firing probe. However the side-firing probe is not essential.
Operation of the solid-state laser in a xe2x80x9cmacropulsedxe2x80x9d mode is more efficient in inducing rapid tissue ablation than a CW laser of the same average power. This is in part because the macropulsing is more efficient in inducing xe2x80x9ccharxe2x80x9d formation, a mild carbonization in which the tissue typically darkens slightly but does not necessarily turn completely black. Although char formation is not essential to efficient rapid ablation it is helpful because the darkened tissue is better at absorbing light. The macropulsed laser is also more efficient and has higher beam quality, with M2 values typically less than 144, than a continuous wave laser with same average output power.
According to a second embodiment of the invention, a method for treating soft tissue comprises the steps of providing a solid-state laser having a laser element positioned to receive pump radiation from a pump radiation source; modulating the pump radiation source to cause the laser element to emit laser light having a pulse duration of between 0.1 milliseconds and 500 milliseconds and an output power exceeding 20 watts; and delivering the laser light to targeted tissue.
According to a third embodiment of the invention, a method for treating BPH comprises the steps of providing a solid-state laser having a laser element positioned to receive pump radiation from a pump radiation source; Q-switching the laser to generate a quasi-continuous wave (CW) beam having an output power exceeding 60 watts; and, delivering the beam to targeted prostate tissue.
According to a fourth embodiment of the invention, a method for treating BPH comprises the steps of providing a solid-state laser having a laser element positioned to receive pump radiation from a pump radiation source such as a laser diode; Q-switching the laser to generate a quasi-continuous wave (CW) beam having an output power exceeding 20 watts with an M2 less than 144; and delivering the beam to prostate tissue.